Support loading local model directories

[dewana-sl]

Summary

Adds local model-directory loading for users who have already downloaded KittenTTS model assets.

Details

• Adds load_from_local(model_path, backend=None).
• Allows KittenTTS("/path/to/model-dir") when the path exists locally.
• Validates config.json, model file, and voices file before constructing the ONNX model.
• Keeps Hugging Face imports lazy so local loading can be imported without the download client path being initialized.
• Documents the local-directory usage in the README.

Addresses #132 and covers the local-cache/repeated-download pain behind #20.

Validation

• python3 -m unittest -q
• Editable install in a fresh virtualenv with declared package dependencies.
• Import smoke for kittentts, KittenTTS, load_from_local, and normalize_text.
• Real inference smoke from a cached local kitten-tts-nano-0.8-int8 snapshot directory.